Am I having a boy or girl?

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You're bound to be curious about your baby's gender, even if you're waiting until the birth to find out. But what if you could tell just by the season in which you conceived, or by what job your partner has? Folk tales are one thing, but what real-life factors may affect the gender of your baby?

Will I have a baby boy?

You may be slightly more likely to conceive a boy if:
You and your partner are living together

One large study of 86,000 parents showed that couples who were living together when they conceived were slightly more likely to have a boy (Norberg 2004).

Just over half (51.5 per cent) of couples living together before conception had a boy, compared with 49.9 per cent of those who lived apart. But why domestic bliss results in more boys is not entirely clear.
You've been living together for less than a year

If your relationship is new and exciting, you'll probably be having lots of sex (Brewis and Meyer 2005). Frequent sex gives male sperm a slightly bigger advantage than usual.

Male sperm are lighter, have smaller heads, and shorter tails (Jongbloet 2004). They can swim more easily through the slightly less hospitable fertile mucus, which coincides with the beginning and the end of your fertile window. This includes the time of ovulation (Jongbloet 2004).

Some experts think that female sperm are more likely to reach the egg in the middle of your fertile period (James 2008).

By having frequent sex, you are more likely to have sex at the start and end of your fertile window, when male sperm have a slight advantage.
You're feeling stressed

Boys tend to be conceived at times of sustained stress, such as during and following a war (Helle et al 2009, Hohmann et al 2010).

One theory is that the mum's high stress levels leads to increases in testosterone and cortisol (Grant 2007, Grant and Irwin 2009). This may lead to changes in the egg, which make it easier for male sperm to penetrate.

Just how this happens isn't clear, but there is still so much we don't know about the exact moment of conception.
You have a higher calorie intake

One study divided 740 British women into groups according to their calorie intake (Mathews et al 2008). Of the women who had the highest energy intake (about 2,413 calories), 56 per cent had boys. Of the women who had the lowest energy intake (about 2,283 calories), only 45 per cent had boys.

Boys are more fragile and are statistically less likely to be conceived, or to survive pregnancy, when food is short. Girl sperm are more robust, so in harder times they may be more able to survive.

Will I have a baby girl?

You may be slightly more likely to conceive a girl if:
You're over 35 years old and the dad-to-be is over 40 years old

The older the mum and dad, the more likely the female sperm are to fertilise the egg (Matsuo et al 2009). This could be due to hormonal influences in the woman as she ages. And in older dads, the female sperm is more likely to win the race. The quality of sperm deteriorates with age and older men produce fewer male sperm.

Or it could be because we tend to have sex less often (Brewis 2005) as we get older. Also, older couples may be more likely to have sex in the middle of the fertile period, when female sperm may have the edge over male sperm.
You suffer badly from pregnancy sickness

A review of 13 studies found that 55 per cent of babies born to women who'd had severe pregnancy sickness were girls (Veenendaal et al 2011). This was compared with 49 per cent in the control group of women who didn't suffer from it. Some experts think that this is due to high levels of the hormone oestrogen, which is linked with pregnancy sickness.
You were born in a tropical climate and conceived there

Fancy a long holiday, somewhere warm?

It's not clear how much time you and your partner would need to spend in a tropical climate for there to be any effect, or if you would need to have been born there, too. But a global survey of birth data showed that more girls are born in the tropics (the region around the equator) than anywhere else in the world (Navara 2009).

One theory is that the hormone melatonin has an effect. Melatonin causes changes in the reproductive systems of animals in response to the length of the day. More females are born when the days are longer. Or it could be that a high temperature alters the survival of male or female sperm in semen.
Your baby was conceived in the winter

However, in some countries, more girls are conceived in winter, and more boys are conceived in the summer (Cagnacci et al 2003, Melnikov and Grech 2003). Farmers have noticed this effect in their animals for many years.

This may be because more viral and bacterial infections are around in the winter. As male sperm and embryos are more fragile, girls may be more able to withstand mums catching infections than boys.
Dad-to-be's job involves pollutants or stress

A few studies have looked at the dad's occupation and the ratio of boys and girls born to them. They found that jobs that involve stress or pollutants seem to result in more girls.

Though they were small-scale studies, which only looked at certain occupations, dads with the following jobs resulted in more girls than average:

professional driver (James 2008)

deep-sea diver (James 2008)

submarine sonar technician (Volk 2004)

flour mill worker (Milham and Ossiander 2008)

high-performance pilot (James 2008)

You conceived after having your ovulation induced hormonally

Some studies carried out among women who used hormones to induce ovulation, such as clomiphene, show that you are slightly more likely to have a girl if artificial hormones are involved (James 2008). However, other experts aren't convinced.
You already have two or more children

It's possible that the more children you have, the more likely you are to have a girl. This may be due to the fact that the more pregnancies you have, the higher your levels of the hormone gonadotrophin (James 1980). Some experts have claimed that these higher hormonal levels could be linked to a higher chance of having a girl.

James WH. 2008. Evidence that mammalian sex ratios at birth are partially controlled by parental hormone levels around the time of conception. J Endocrinol 198:3-15 joe.endocrinology-journals.org [pdf, accessed September 2012]

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